This function tries to determine the full path to a command line program
using which on Unix/Linux/OS X and win32api on Windows. Most of the
time it will use the version that is first on the users PATH. If
cmd is python return sys.executable.

Warning, don’t use this to find IPython command line programs as there
is a risk you will find the wrong one. Instead find those using the
following code and looking for the application itself: